.\"
.\" Copyright (C) 2022 Alexander Chernikov <melifaro@FreeBSD.org>.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd November 1, 2022
.Dt GENETLINK 4
.Os
.Sh NAME
.Nm genetlink
.Nd Generic Netlink
.Sh SYNOPSIS
.In netlink/netlink.h
.In netlink/netlink_generic.h
.Ft int
.Fn socket AF_NETLINK SOCK_DGRAM NETLINK_GENERIC
.Sh DESCRIPTION
The
.Dv NETLINK_GENERIC
is a "container" family, used for dynamic registration of other families
belonging to the various subsystems.
These subsystems provide a string family name during registration and
receive a dynamically-allocated family id.
Allocated family identifiers are then used by applications to get access to
functions provided by that subsystem via netlink.
There are standard methods for resolving string family names to family
identifiers.
A similar mechanism works for the notification groups provided by those
families.
.Pp
All generic netlink families share a common header:
.Bd -literal
struct genlmsghdr {
	uint8_t		cmd;		/* command within the family */
	uint8_t		version;	/* ABI version for the cmd */
	uint16_t	reserved;	/* reserved: set to 0 */
};
.Ed
The family id is encoded in the
.Dv nlmsg_type
of the base netlink header.
The
.Va cmd
field is the command identifier within the family.
The
.Va version
field is the command version.
.Sh METHODS
The generic Netlink framework provides the base family,
.Dv GENL_ID_CTRL
("nlctrl") with a fixed family id.
This family is used to list the details of all registered families.
.Pp
The following messages are supported by the framework:
.Ss CTRL_CMD_GETFAMILY
Fetches a single family or all registered families, depending on the
.Dv NLM_F_DUMP
flag.
Each family is reported as
.Dv CTRL_CMD_NEWFAMILY
message.
The following filters are recognised by the kernel:
.Pp
.Bd -literal -offset indent -compact
CTRL_ATTR_FAMILY_ID	(uint16_t) current family id assigned by kernel
CTRL_ATTR_FAMILY_NAME	(string) family name
.Ed
.Ss TLVs
.Bl -tag -width indent
.It Dv CTRL_ATTR_FAMILY_ID
(uint16_t) Dynamically-assigned family identifier.
.It Dv CTRL_ATTR_FAMILY_NAME
(string) Family name.
.It Dv CTRL_ATTR_HDRSIZE
(uint32_t) Family mandatory header size (typically 0).
.It Dv CTRL_ATTR_MAXATTR
(uint32_t) Maximum attribute number valid for the family.
.It Dv CTRL_ATTR_OPS
(nested) List of the operations supported by the family.
The attribute consists of a list of nested TLVs, with attribute values
monotonically incremented, starting from 0.
The following attributes are present in each TLV:
.Bl -tag -width indent
.It Dv CTRL_ATTR_OP_ID
Operation (message) number.
.It Dv CTRL_ATTR_OP_FLAGS
Operation flags.
The following flags are supported:
.Bd -literal -offset indent -compact
GENL_ADMIN_PERM		requires elevated permissions
GENL_CMD_CAP_DO		operation is a modification request
GENL_CMD_CAP_DUMP	operation is a get/dump request
.Ed
.El
.It Dv CTRL_ATTR_MCAST_GROUPS
(nested) List of the notification groups supported by the family.
The attribute consists of a list of nested TLVs, with attribute values
monotonically incremented, starting from 0.
The following attributes are present in each TLV:
.Bl -tag -width indent
.It Dv CTRL_ATTR_MCAST_GRP_ID
Group id that can be used in
.Dv NETLINK_ADD_MEMBERSHIP
.Xr setsockopt 2 .
.It Dv CTRL_ATTR_MCAST_GRP_NAME
(string) Human-readable name of the group.
.El
.El
.Ss Groups
The following groups are defined:
.Bd -literal -offset indent -compact
"notify"	Notifies on family registrations/removal.
.Ed
.Sh SEE ALSO
.Xr netlink 4
.Sh HISTORY
The
.Dv NETLINK_GENERIC
protocol family appeared in
.Fx 13.2 .
.Sh AUTHORS
The netlink was implemented by
.An -nosplit
.An Alexander Chernikov Aq Mt melifaro@FreeBSD.org .
It was derived from the Google Summer of Code 2021 project by
.An Ng Peng Nam Sean .
